The present invention relates to anti-lock braking systems and in particular to an anti-lock braking system which incorporates an integrated modulating valve and brake pressure reducing pump, actuated by an electronic control system responsive to rotational behavior of a vehicle's wheels, the system being particularly applicable to anti-lock control of the rear wheels of a vehicle.
Anti-lock braking systems are well known. Typically, prior art anti-lock braking systems have evolved from mechanical to sophisticated electronically-controlled devices which sense the rotational behavior of a vehicle's wheels during braking and assume control of braking of the vehicle upon sensing an imminent skid condition to automatically modulate the application of braking pressure to avoid wheel-lock. Typically, these systems incorporate a power boosted master cylinder or a full power hydraulic system to produce the required pressurized braking fluid. The systems are normally divided into a plurality of hydraulic circuits with braking of the front wheels of the vehicle being individually controlled and the rear wheels of the vehicle being controlled either individually or as a pair. Such systems can be highly effective. Such systems are, however, complex and expensive, a factor which reduces their availability and widespread implementation.
There exist numerous applications for anti-lock braking systems in which substantial improvements in the braking performance of the vehicle can be enhanced without the requirement of controlling three or more wheels of the vehicle. Such applications typically include pick-up trucks and vans. In these applications, due to the substantial variation in loading of the rear wheels of the vehicle, it is possible to have a vehicle that is lightly loaded be very subject to locking of the rear wheels. Various brake pressure proportioning devices have been proposed to alleviate this problem but such devices have been less than fully effective, due to substantial variations in friction materials and other variable parameters of the braking system.
There therefore exists a need for a simple, low cost anti-lock braking system which is adaptable for applications such as pick-up trucks and vans which can substantially reduce the potential of rear wheel lock with a minimum of cost and complexity.
There also exists a need for such a system which can be used with braking systems incorporating vacuum boosted modulators, and any of various hydraulically boosted systems.
Broadly, the present invention is an anti-lock braking system which comprises a master cylinder means for generating pressurized braking fluid in a hydraualic circuit which includes at least one wheel cylinder. Means are provided for sensing the rotational behavior of a vehicle wheel and generating a signal corresponding thereto. A control circuit is connected to the sensor means for receiving these signals and generating control signals in response to an incipient wheel-lock condition. A brake pressure modulator is operatively connected to the hydraulic circuit between the master cylinder and the wheel cylinder. The pressure modulator includes a pump for pumping fluid from the wheel cylinder to the master cylinder, and a pressure reducing means connected between the pump means and the master cylinder for producing a pressure drop thereacross in response to the flow of fluid therethrough. An isolating valve is provided, the valve having an inlet port, wheel cylinder outlet port, a pump outlet port, and a control port. The ports are connected to the master cylinder, wheel cylinder, the inlet of the pump means, and the input of the pressure reducing means, respectively. The isolating valve normally establishes fluid communication between the inlet, wheel cylinder, and pump inlet ports and includes an element operable in response to a pressure drop across the pressure reducing means to isolate the inlet port from the wheel cylinder and pump inlet ports. The pump means is responsive to the signals from the control means to pump fluid from the wheel cylinder to the master cylinder to thereby reduce the pressure of braking fluid therein.
It is therefore an object of the invention to provide an improved anti-lock braking system.
Another object of the invention is to provide an anti-lock braking system of reduced cost and complexity.
Still another object of the invention is to produce an anti-lock braking system particularly well suited for use in preventing lock of the rear wheels of a vehicle.
Another object of the invention is to provide an anti-lock braking system which includes a self-contained electrically operated pump and an isolating valve operable in response to signals from a control circuit and fluid flow from the pump, respectively, to pump fluid from a wheel cylinder and reduce the pressure therein.